458 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



vances the suggestion that marine current scour is responsible for many 

 of the local breaks in sedimentation, saying that "marine waters may not 

 only deposit sediment, but may also prevent deposition, or even remove 

 a deposit previously made." However, as shown on pages 362 to 375, 

 marine currents can not possibly account for more than a very small 

 percentage of the imperfections in the sedimentary column whose exist- 

 ence is proved beyond any reasonable doubt by faunal and physical evi- 

 dence. So far as I can see, no other conclusion is possible under the 

 circumstance than that land surfaces frequently existed in Paleozoic 

 time which, on resubmergence, left no depositional evidence of subaerial 

 decay and erosion. 



But, after all, is the absence of such evidence to be regarded as extra- 

 ordinary? I think not; and most certainly I can not concede that the 

 condition is impossible, nor even improbable. Considering the kind of 

 lands that must have prevailed in regions where such obscurely indicated 

 emergences, the Eopaleozoic more particularly, are most common, the 

 sedimentary record in the adjacent continental seas is as complete as it 

 could be made under the circumstances. Let us, then, seek to interpret 

 geologic history according to the facts as we find them and not solely 

 according to criteria suiting conditions prevailing today. 



The mere absence of distinctive shore deposits surely does not out- 

 weigh all the other evidence on which small low lands and shallow seas 

 is inferred. This negative evidence goes no farther than to suggest that 

 the lands were too low and the seas too shallow to favor the development 

 and accumulation of such material. But there are other lines of reason- 

 ing that may be brought to bear on this problem. 



That the Paleozoic interior lands were as a rule very low may be ac- 

 cepted as an established fact. Erosional processes consequently affected 

 them only to a viery small, not to say negligible, extent. There must 

 have been some surface decay, but unless we assume that the lands were 

 generally clothed with abundant plant growth — a most improbable condi- 

 tion prior to the Devonian — even this can not have been very great. Soil 

 must have formed slowly and, with few or no vascular plants to facilitate 

 its downward extension, it must also have required a long time to attain 

 any considerable depth. Doubtless, too, its particles were very finely 

 divided. Finally, study of Paleozoic unconformities shows that surface 

 inequalities that are evidently due to irregularities in superficial rock 

 decay became common only after the Silurian. 



With slow submergence of such a land the advancing sea must have 

 taken nearly the whole of the residual mantle into suspension. Some of 

 it may have been swept into whatever hollows there were, but most of the 



